Electro-catalytic photography



Nov. 28, 1967 J, J. ROBILLARD ELECTRO-GATALYTIC PHOTOGRAPHY Filed June 27 2 Sheets-Sheet l II II I I I I r/ I /l 1 1 1/ 1/ 1/ I TRANSPARENT ELECTRODE ICONDUCTOR ELECTRICAL SOU RCE ELECTRODE I NVENTOR JEAN J. ROBILLARD ofidfi 9129a!) ATTORNE Y5 Nov. 28, 1967 J. J. ROBILLARD 3,355,290

ELECTED-CATALYTIC PHOTOGRAPHY Filed June 27, 1962 2 Sheets-Sheet 2 FIG. 5

PAPER ADVA CE CONTROL FIG. 6

INVENTOR. JEAN J. ROBILLARD ATTORNEYS United States Patent 3,355,290 ELECTRO-CATALYTIC PHOTOGRAPHY Jean J. Robillard, 210 Harris Ave., Woonsocket, RI. 02903 Filed June 27, 1962, Ser. No. 205,600 13 Claims. (Cl. 961.5)

The present invention relates generally to a dry photographic process and apparatus which utilize a sensitized medium to make a permanent visible indication of received radiation such as visible light or other radiation, and particularly relates to such process and apparatus wherein the recording medium comprises a semiconductive oxide in which a change in state of oxidation is obtained by the selective introduction of minute amounts of a catalyst in the semiconductor oxide lattice.

This application is a continuation-in-part of my prior co-pending application, Ser. No. 182,864, for Electro- Catalytic Recording, filed Mar. 27, 1962.

Numerous technological areas have been explored in an attempt to develop a practical photographic or photocopy process which is both dry and rapid and is otherwise comparable with the commonly used wet processes. Although numerous dry processes have been developed, they generally have one or more major disadvantages as compared to wet processes so that the latter continue to be used in the great majority of instances in spite of their obvious disadvantages.

Examples of dry processes include thermal processes in which a chemical change is induced by heat and tem perature increase, and electro-static processes in which a fine powder is selectively deposited in accordance with an image and fused or otherwise secured in position to form a permanent record. The thermal process has the disadvantage of impermanency in the presence of sustained moderately high temperatures while the electro-static process requires the presence and distribution of at least one powdered material which creates practical inconveniences almost as great as in the case of wet processes.

According to the present invention, a dry process with quite good speed (sensitivity) is provided in which a substantially permanent image is formed by virtue of the electrical injection of a minute quantity of catalytic material into selected portions of a recording medium according to an image projected thereon. The electro-catalytic process is simple to carry out, for example by passing the recording medium under a roller or other electrode coated with the catalytic material and maintained at a low electric potential with respect to the recording medium.

In addition to providing the above described features and advantages, it is an object of the present invention to provide an electro-catalytic photographic process wherein catalytic ions are electrically injected in minute quantities into a recording medium, the injection of such ions being controlled by a photoconductive material associated with the medium.

It is another object of the present invention to provide an electro-catalytic photographic recording medium comprising a semiconductor material susceptible of a change in state accompanied by a change in color upon the introduction of minute amounts of a catalyst in the semiconductor and also comprising a photoconductive material for control of the injection of the catalyst by an electric field.

It is still another object of the present invention to provide photographic apparatus including means for producing an image of received radiation, a recording medium including a semiconductor material sensitive to the introduction of minute amounts of catalyst by an electric field, photoconductive material associated with the medium for controlling the introduction of the catalyst in accordance with the image projected on the recording medium, and mean for generating an electric field through said medium.

Other objects and advantages of the present invention will be apparent from a consideration of the following description in conjunction with the appended drawings in which:

FIGURE 1 is a diagrammatic illustration of an electrocatalytic photographic recording medium according to the present invention;

FIGURE 2 illustrates an alternative form of electrocatalytic photographic recording medium according to the present invention;

FIGURE 3 is an isometric view, partially schematic, of photographic apparatus according to the present invention;

FIGURE 4 is a diagrammatic edgewise view of the image recording portion of the apparatus of FIGURE 3;

FIGURE 5 is a schematic diagram of photocopy apparatus according to the present invention; and

FIGURE 6 is a lattice diagram of an exemplary semiconductor material utilized in the present invention and presented to aid in the explanation of a theory of operation of the invention.

Referring now to FIGURE 1, photosensitive recording apparatus 11 is shown comprising a transparent electrode 12 which may be formed of conductive glass such as NESA glass or may comprise a transparent nonconductive sheet having a conductive film deposited thereon.

The second layer 13 is a photoconductive layer which may be formed of cadmium selenide, zinc oxide, or antimony trisulfide. Layer 14 is a catalyst layer which acts as a source of catalytic ions and may comprise for example a thin layer of silver or a silver compound. The layer 14 will have high surface resistance so that there will be negligible current flow along the surface of layer 14, but the transverse resistance will be low and present little resistance to current flow through the very thin layer.

Layer 15 is a semiconductor material having special properties which will later be described in more detail. Layer 15 may comprise impurity-doped cerium oxide (c602)- I Layer 16 is a second electrode which need not be transparent and may comprise aluminum foil as an example. The layer 16 may alternatively be a conductive layer on a reinforcing backing such as plastic or paper. The layer 16 may thus be transparent if it is desired to produce a transparency form of record.

Electrode 12 is connected by lead 17 to an electrical source 19. Another lead 18 connects electrical source 19 to the electrode 16. An electric potential difference may thus be provided between electrodes 12 and 16 which may be on the order of 10 volts. Obviously the electrical source may be electrically connected to or disconnected from electrodes 12 and 16 to render the recording apparatus operative or inoperative as may be desired (and such a switching arrangement could thus take the place of a shutter).

The apparatus of FIGURE 1 operates as follows. When an image is projected onto layer 12 (which image may be a shadowgraph as well as an optically focused image), the layer 12 transmits the radiation so that it falls on photoconductor layer 13 rendering those portions on which radiation falls-relatively more conductive than the dark portions of layer 13. Thus different conductivity paths will be provided in the photoconductive layer 13 according to the distribution of the image brightness. These different conductivity paths will provide electrical currents through the sandwich as a result of the electric potential applied between electrodes 12 and 16. The current intensity will vary according to the conductivity of layer 13 and hence according to the local brightness of the image. Catalytic ions from layer 14 will be transported into semiconductor layer in numbers corresponding to the current flux in each area of the surface of layer 15 (the polarity of electrodes 12 and 16 is determined to transport the ion in the direction from layer 14 to layer 15). In this case the image is negative, but other materials yield a positive image.

The transport of catalyst into the semiconductor layer 15 triggers a change in state of oxidation of the semiconductor, usually an oxide, and thereby provides a change in color. The nature of this chemical reaction will be more fully described hereinafter.

In a usual case, the electrode 12 in FIGURE 1 will not be permanently sandwiched with the remaining layers but will constitute a plate against which is pressed a permanent sandwich comprising some or all the remaining layers.

In one form the electrode 12 and the photoconductive layer 13 may be united in a permanent plate while the remaining layers 14, 15 and 16 constitute the recording paper.

Of course, the complete sandwich as represented in FIGURE 1 could be utilized as a unitary photographic recording plate in which case an image would appear immediately upon application of an electric field between electrodes 12 and 16 which would be visible through the transparent electrode 12, the photoconductive layer 13 and the thin catalyst layer 14. In such case photoconductive layer 13 and catalyst layer 14 will decrease somewhat the ultimate contrast visible in the image formed in semiconductor layer 15. The apparatus of FIGURE 2 which will be later described eliminates this diminution of contrast.

It should be pointed out with respect to the photographic mechanism of the recording medium of FIG URE 1 that it is necessary that the catalyst atoms or molecules in layer 14 be electrically charged in order that they may be transported through the sandwich with an electric current. The present invention provides that these catalytic atoms or molecules be ionized. Thin metallic layers obtained by vacuum deposition or similar processes have a very high chemical activity for thicknesses lower than 0.1 micron and as a consequence they are always oxidized. Furthermore the bond between the oxygen atoms and the metal atoms is much weaker than in a bulk oxide or thick layer of oxide. This is due to the fact that the resultant of the forces exerted on a particular molecule of oxide in a direction perpendicular to the film does not balance the one exerted in a direction parallel to the film. As a result, if an electric field is applied perpendicular to the film, the oxide molecule will easily dissociate and provide a metal ion which is transportable by the electric current (Langevin theory).

Another requirement for optimum operation of the recording media of FIGURE 1 is that there be adequate transverse conductance in layers 13, 14, and 15 in order that a low electrical potential will produce sufficient electric current to transport the small amount of catalyst required to form a visible image in semiconductor layer 15.

In a further alternative form of apparatus according to FIGURE 1, the structure represented could be formed, in two parts, the first containing transparent electrode 12', photoconductor layer 13 and catalyst layer 14, while the other part would be the recording paper and would comprise conductive electrode 16 and semiconductor layer 15. The paper comprising layer 16 and 15 would then be firmly pressed against the plate comprising electrode 12, layer 13 and layer 14 during the exposure and application of the electric field. After exposure the paper comprising layers 15 and 16 would be separated and would constitute a permanent record. For example, elements 12, 13 and 14 may constitute a permanent element of a camera or other photographic apparatus to be utilized in conjunction with photographic paper or film comprising layers 15 and 16.

Such apparatus is illustrated in FIGURE 3 in which a camera 25 is provided with a conventional enclosure 26, a back 27, a view finder 28 of conventional form, and a conventional lens and shutter assembly 29. A photographic plate 31 is supported on back 27 by suitable brackets 32 or the like.

A source of electric potential 33 is provided which may comprise a dry cell battery of 10 to 15 volts, for example, and which is electrically connected by leads 34, 35 and 36 to the photographic recording plate 31.

In addition to conventional controls 37 for the camera lens opening, shutter, and the like, a control button 38 may be provided for the electric potential supplied to the recording plate.

The recording plate 31 may be of the form illustrated in FIGURE 1 and more specifically may comprise two parts consisting of recording paper and exposing plate, the paper comprising layers 15 and 16 in FIGURE 1, and the exposing plate comprising layers 12, 13 and 14. The photographic plate 31 may alternatively comprise other forms of the electrocatalytic photographic recording media according to the present invention.

A further alternative form of recording medium according to the present invention is illustrated in FIGURE 2. In FIGURE 2 the photographic recording medium 21 comprises a photoconductive layer 23 which has the property of retaining conductivity due to exposure to light for a significant time after the exposure has taken place. As an example of such a photoconductive medium, zinc oxide has this property. Other photoconductors which involve chemi-sorption also have this property. The photoconductor layer 23 is deposited on an electrode 24 which may have the same characteristics as electrode 16 in FIG- URE l. A semiconductor layer 22 is deposited on top of the photoconductive layer 23 and the semiconductor layer 22 may have the properties described for layer 15 in FIG- URE 1.

In comparing the media of FIGURE 2 with that of FIGURE 1 it will be noted that the transparent electrode 12 is eliminated as well as the thin catalyst layer 14; the photoconductive layer and the semiconductor layer are inverted.

The recording media 21 of FIGURE 2 is particularly useful in photocopy applications and may be utilized in apparatus such as that illustrated in FIGURE 5, for example. Photocopy apparatus 41 comprises a lens 42 for focusing an image of a document or other subject to be copied. The image from lens 42 is focused on a length of paper 21 supplied from a supply reel 43. The paper 21 runs between an image fixing roller 45 and a drive roller 46.

The drive roller 46 is driven through pulleys 47 and 48 and belt 49 by an electric motor 51. Suitable controls 52 are provided to control the paper advance by actuation of motor 51. Rollers 45 and 46 are formed of conductive material and are electrically connected by leads 53 and 54 to a source of electric potential.

Image fixing roller 45 is coated with a thin layer of a catalyst material, such as silver.

The operation of the apparatus of FIGURE 5 with the recording media 21 of FIGURE 2 is as follows:

An image is projected by lens 42 onto the recording medium 21 for the exposure thereof. Immediately after exposure, the exposed area is transported between rollers 45 and 46 between which is maintained an electric potential as indicated. Photoconductive layer 23 retains its photoconductivity pattern from the exposure while it is passed between rolls 45 and 46. Accordingly, a corresponding current pattern will flow through the area between the two rollers and a controlled amount of catalyst material will be transported from the upper roller into the semiconductor oxide layer according to the conductivity pattern. It will be appreciated that the process and apparatus of FIGURE is unusually simple and effective and eliminates many complexities of existing photocopy methods.

It will be noted that the photographic recording medium 21 of FIGURE 2 is arranged so that the light forming the image must pass through semiconductor layer 22 before reaching photoconductor layer 23. Although this reduces somewhat the sensitivity of the recording medium the sensitivity is still ample for photocopy purposes.

The arrangement of layers in the photographic recording medium 21 has the advantage that the exposed layer 22 is the image recording layer, thus obviating the necessity for viewing the image through another layer which would cut down the available contrast.

In previous examples it has been contemplated that the photoconductive layer and the semiconductor layer would be separately deposited. However, in some cases it will be desired to combine the photoconductive material and the semiconductor material in a single layer formed of a dispersion of both materials. Such a system would operate in much the same way as previously described. The path of current would be produced by the photoconductive material dispersed in the combined layer.

'With the combined layer the sensitivity of the paper will be higher since the light will not have to pass through the image-forming layer to reach the light sensitive layer.

Those of skill in the art will appreciate and be able to carry out the invention from the foregoing description. However, further details of the operation of the invention and of the method of production of the electrocatalytic photographic recording media, may be of assistance in the practice of the invention.

As regards the mechanism of the change in oxidation state due to the introduction of a small amount of catalyst in the semiconductor oxide material, two categories should be considered.

In the first category only the formation of activation centers proportional to the number of catalyst atoms introduced into the system is involved. These activation centers are invisible and form a latent image similar in these respects to the one obtained with silver halide photography. This latent image can be developed by further' exposure to a radiation of short wavelength such as ultraviolet light.

In the second category, the change in oxidation state, accompanied by the change in color, takes place spontaneously upon the introduction of the catalyst under influence of an electric field. This involves a chain reaction in which the number of oxide molecules affected is far greater than the number of atoms of catalyst introduced into the system.

It will be apparent that the second category process, which does not involve further exposure to radiation to develop'the latent image is somewhat preferable. However, the requirement for exposure to further radiation is usually met simply by exposure to daylight, flourescent light, or other low level illumination with some ultraviolet component. Thus the requirement for further exposure to radiation is not a serious disadvantage.

A list of different semiconductor oxides that are particularly suitable for this process is given hereafter together with the change in color which accompanies the change in oxidation state.

TABLE I Materials Color M00 M00 White Gray PbOz Pb O White Black TiOz T10 White Black V 05 V204 Yellow Blue T3205 T3204 White Black CeOz CeO White Black Blgoa B1205 Yellow Black ch03 OrO Green Black C00 00304 Green Black MnO M11203 Green Black NiO N i203 Green Black T302 TaO White Black SnOz S110 White Black W03 W02 Yellow Brown In order to enhance the semiconductive properties of the semiconductor material it is often necessary to dope this material. The doping material and procedure will vary somewhat with the oxide considered as will be understood by those skilled in the art. The doping material and procedure is determined from the semiconductors energy band structure and other solid state properties.

By way of example the preparation of a coating of titanium dioxide will be described in some detail. In the formulation of such coating commercially pure titanium dioxide may be used. This material is readily available and is utilized in the manufacture of paint, for example. Titanium dioxide of this grade will normally be found to have many crystal defects which is desirable for use in formulating an electro-catalytic coating according to the present invention. If desired, an X-ray analysis of the titanium dioxide to be used may be made to assure that it has the desirable crystal defects.

The titanium dioxide is sensitized by doping to introduce impurities in minute quantities. A sodium hydroxide solution may be utilized for doping the titanium dioxide thus introducing sodium ions as impurities in the titanium dioxide crystals.

The following procedure may be followed to prepare the coating. Commercially pure titanium dioxide powder is ball milled for approximately one week to a grain size of about .1 micron.

The finely divided titanium dioxide powder is then agitated for several hours in a solution of sodium hydroxide (approximately 10%). This solution may be obtained by adding the appropriate amount of sodium hydroxide to the water in which the finely divided titanium dioxide was milled. The agitation in the sodium hydroxide solution results in the doping of the titanium dioxide which may thereafter be dried in a vacuum oven.

In order that the titanium dioxide may be coated on a foil sheet or other base it is dispersed in a binder. The binder may be of aqueous or non-aqueous type.

As an example of a non-aqueous binder, Pliolite S-7 manufactured by Goodyear may be utilized and the powder mixed with commercial solution of Pliolite in a proportion of three parts powder to one part Pliolite. All proportions set forth herein are by weight unless otherwise specified.

The Pliolite-powder mixture is diluted with toluene to the proper viscosity for convenient coating (about the same viscosity as ordinary oil base house paint is convenient).

Rather than the non-aqueous binder described above, an aqueous binder may be utilized such as polyvinyl alcohol. A 10% solution of ammonium nitrate in water is used to dilute the polyvinyl alcohol. Ammoniumnitrate solution rather than water provides conductivity in the coating. The original proportions of polyvinyl alcohol and the diluent may be 7% polyvinyl alcohol, 93%

- diluent. The binder is again mixed with powder in pro- After the coating has been prepared with a desired binder as described above it may be coated on foil or other conductive paper. Any suitable coating process may be utilized, for example a knife coating machine may be used to coat a layer of the electro-catalytic material onto a base. The thickness of the coating is not highly critical and may vary from microns to as much as 2 mils in some instances. Generally a thin coating is desirable, particularly if very low potentials are to be utilized for fixing an image.

Another example of a formulation of electro-catalytic coating material according to the present invention is based upon the use of stannic oxide (SnO The preparation of a stannic oxide electro-catalytic coating is substantially similar to that previously described for titanium dioxide. The doping solution may be 5% sodium acetate solution, an aqueous binder may be utilized as previously described, and it may be found desirable to add to the binder a minute quantity of surfactant, for example Igepal. The finely divided stannic oxide may be mixed with a binder in proportions of 50 grams of oxide per 100 cc. of binder.

The above described stannic oxide coating is characterized by very black recording on white background at a potential of approximately volts. This coating also has the advantage of greatly reducing or eliminating the necessity for illumination by ultraviolet or other radiation to complete the recording process.

As a further example a coating may be formed of CeO by a process substantially as previously described for titanium dioxide. CeO is doped by mixing the material with 10% solution of sodium hydroxide and agitating in this solution for a period of six (6) hours.

Silver is an appropriate catalyst for use in connection with cerium oxide and also with titanium dioxide and stannic oxide.

As stated in my prior copending application, other catalysts may be preferred with other electro-sensitive compounds, for example, tantalum oxide may be used with a copper or manganese catalyst, lead oxide may be used with a nickel catalyst. In addition, a thallium catalyst may be used with titanium dioxide, cerium dioxide or germanium dioxide as an alternative to the silver catalyst.

The foregoing examples of formulation and preparation of electro-sensitive coatings are given by way of example only and subject to considerable modification. In particular the doping of crystals with impurities is well known, for example, in the art of preparation of semiconductor materials, and these known techniques are applicable to the preparation of electro-catalytic coatings according to the present invention. For example, sodium acetate may be utilized as a medium for doping the titanium dioxide with sodium ions rather than the sodium hydroxide previously described.

To present a better understanding of the catalytic reduction involved in the image forming process, the mechanism of the action will be explained with reference to a specific example, cerium oxide.

The catalytic reduction involved in the image forming process is based on the control of the number of ions of deviating valency in an ionic crystal by incorporation of impurity ions of a certain type into the lattice.

In the original cerium oxide a small amount of CeO is always present along with the predominate CeO This small amount of CeO plays an important role in the initiation of further reduction. The introduction of silver ions into the existing CeO lattice will control the number of Ce ions. The smaller charge of the Ag+ ions balances the excess charge of the Ce ions without the simultaneous introduction of vacancies in the cation lattice. The composition of the resulting material may be designated (Ag ,Ce Ce +,,)0.

The defect center may be described as an impurity cation of lower relative charge on a cation site plus a positive hole bound on a neighboring host cation. In contrast to stoichiomctric CeO the silver containing material shows a considerable increase in electrical conductivity of the p-type. The silver catalized cerium oxide has much the same properties as CeO prepared under slightly reducing conditions, but in the latter material, there is no way of estimating or controlling the concentration of defects. The incorporation of a cation site of an impurity cation of higher charge than the host cation can stabilize a lower valence state of the host cation. The situation discussed here is illustrated in FIGURE 6. When occupied by a silver ion the defect center may be described as an impurity cation of higher relative charge on a cation site plus a quasi-free electron bound to a neighboring host cation.

A condition apparently necessary for the mechanism involved in the present electrocatalytic photographic process is that the impurity cation should be of much the same size as the host cation. The principle is different from usual semiconductors in that it allows the creation of electrical defects without the creation of a corresponding number of vacancies in the lattice.

In the first category of oxides previously discussed (those requiring development by exposure to further radiation such as ultraviolet light) the existence of electrical defects in the crystal lattice will provide trapping levels in the energy diagram at different depths under the conduction band. The position of these levels in the energy diagram is such that the absorption of a quantum of light in the ultraviolet band will create free electrons which will be trapped at the existing trap-ping centers. As a result, the trapping center will provide a local concentration of negative charges. In addition to this electron trapping, an ionic process takes place in the CeO emulsion, where CeO ions are displaced from the lattice and are moved forward to the negative trapping centers where they discharge themselves.

The motion of the ions is of two kinds: direct motion from one interstitial position in the lattice to another, and the filling up of the vacated sites by several ions in the lattice producing new vacant sites so that positive holes migrate. As the development proceeds, more electrons are provided to the trapping levels by the absorbed UV light and the reduced state of oxide is built up accordingly. The oxygen atoms released are diffused to the surface or remain interstitially.

In the second category of materials, the development of the image takes place immediately upon the introduction of the catalyst into the oxide layer without absorption of UV light for the development. The mechanism is as follows: the trapping of an electron by the trapping centers as mentioned previously, tends to hasten the difiusion of 0 ions away from their original sites and leave an equivalent number of cations adjacent to the reduced ion. These cations with anion vacancies are added to the reduced ion neutralizing the trapped electron, and so on. This represents the autocatalytic phase of the reaction which is responsible for amplification and self development of the image.

A study of the reduction mechanism of different oxides, together with their" band structure and semiconductivc properties will determine whether an autocatalytic image forming reaction is possible. When this possibility exists, emulsions can be made which will not require UV exposure for development.

Examples of oxides which do not require any ultraviolet development are tin oxide SnO tantalum oxide Ta O and tellurium oxide TeO The'production of electrocatalytic photographic recording media involves, in addition to the formulation and coating of semiconductor layers as described, the forming of layers of photoconductor material according to known techniques which will not be described and the deposition of catalyst layers also by known techniques and preferably by vacuum deposition.

The theoretical explanation of the operation of the photographic process according to the present invention is believed correct, but it should be noted that the utility and operability of the process and apparatus is not predicated on the accuracy of the theory presented but is rather based upon actual performance and results of the process and apparatus. Likewise, the theory presented is not intended as a limitation on the scope of the invention.

In addition to the modification of the invention illustrated and suggested herein, other variations and modifications will be apparent to those of skill in the art, and it is accordingly desired that the scope of the invention not be limited to those embodiments specifically illustrated or suggested, but that the scope of the invention be defined by reference to the appended claims.

What is claimed is:

1. An electro-catalytic photographic recording medium comprising a semiconductor oxide having two oxidation states of different color, said oxide being substantially in the higher state of oxidation and doped with an impurity in a quantity to render said semiconductor oxide sensitive to the introduction of a relatively small number of metal ions of a solid catalyst with an electric field to change said oxide by catalytic action from the higher to a lower state of oxidation, and a photoconductive material distributed over a surface substantially coextensively with said oxide.

2. A coating as claimed in claim 1 wherein said oxide comprises TiO 3. A coating as claimed in claim 1 wherein said oxide comprises Ta O 4. A coating as claimed in claim 1 wherein said oxide comprises CeO 5. A recording medium according to claim 1 wherein said oxide comprises SnO 6. An electro-catalytic photographic recording medium comprising a semiconductor layer having two oxidation states of different color, said semiconductor being substantially in the higher state of oxidation and doped with an impurity to render said semiconductor layer sensitive to the introduction of a relatively small number of metal ions of a solid catalyst by an electric field to change said semiconductor from the higher to a lower state of oxidation, a photoconductive material and a layer of solid catalytic material each distributed over a surface substantially coextensively with said semiconductor layer, said catalytic material in direct contact with said semiconductor layer and two electrodes for impressing an electric field through said semiconductor layer, said photoconductive material and said catalytic material, one of said electrodes being at least partially transparent.

7. An electro-catalytic photographic recording medium comprising a semiconductor oxide layer having two oxidation states of difierent color, said oxide being substantially in the higher state of oxidation and doped with an impurity in a quantity to render the coating sensitive to the introduction of a relatively small number of ions of a solid catalyst with an electric field of less than 10,000 volts per cm. to change said oxide from the higher to a lower state of oxidation, 21 photoconductive material distributed over a surface substantially coextensively with said oxide, a layer of catalytic material in direct contact with said oxide layer and two electrodes for impressing an electric field through said oxide layer, said photoconductive material and said catalytic material, one of said electrodes being at least partially transparent.

8. The method of producing a substantially permanent visible photographic record characteristic of a radiation image comprising the steps of projecting said image on a sensitized surface comprising a semiconductor material having two oxidation states of different color and a photoconductive material, said semiconductor being substantially in the higher state of oxidation and doped with an impurity in a quantity to render said semiconductor material sensitive to the introduction of a relatively small number of ions of a solid metal catalyst, placing a solid catalyst source in contact with said surface and impressing an electric field throughout said surface to inject said metal ions of said catalyst into said surface in accordance with the conductivity of said photoconductive material causing said semiconductor material to convert to said lower oxidation state in selected areas thereof.

9. The method of claim 8 wherein said electric field is impressed a substantial interval after the projection of said image on said surface.

10. The method of producing a substantially permanent visible photographic record characteristic of a radiation image comprising the steps of projecting such image on a sensitized surface comprising cerium oxide and a photoconductive material, said cerium oxide being substantially in a higher state of oxidation, placing a solid catalyst source in contact with said surface, said solid catalyst comprising silver, and impressing an electric field throughout said surface to inject ions of said catalyst into said surface in accordance with the conductivity of said photoconductive material causing said cerium oxide to convert to said lower oxidation state in selected areas thereof.

11. The method of producing a substantially permanent visible photographic record characteristic of a radiation image comprising the steps of projecting such image on a sensitized surface comprising an oxide of tin and a photoconductive material, said oxide of tin being substantially in a higher state of oxidation, placing a solid catalyst source in contact with said surface, said solid catalyst comprising silver, and impressing an electric field throughout said surface to inject ions of said catalyst into said surface in accordance with the conductivity of said photoconductive material causing said oxide of tin to convert to said lower oxidation state in selected areas thereof.

12,. A process, according to claim 8, wherein said metal ions are selected from a class consisting of silver, copper, manganese and thallium.

13. The method of producing a substantially permanent visible photographic record characteristic of a radiation image comprising the steps of (a) projecting said image on a photoconductive material to provide conductivity paths in the photoconductive material according to the distribution of the image brightness, and (b) impressing an electric field through said photoconductive material and a semiconductor material to inject metal ions from a solid catalyst source into said semiconductor material in accordance with the conductivity of said photoconductive material causing said semiconductor material to convert to a lower oxidation state in selected areas thereof, said semiconductor material having two oxidation states of substantially contrasting color with the lower oxidation state being intensely colored and said semiconductor material being doped with an impurity in a quantity to render said semiconductor material sensitive to the introduction of a relatively small number of said metal ions.

References Cited UNITED STATES PATENTS 2,319,765 5/1943 Talmey 96-1 3,010,883 11/1961 Johnson et al 204-18 3,041,166 6/1962 Bardeon 961 3,082,085 3/1963 Miller et al 961 3,085,051 4/1963 Hamm et a1. 96-1 3,088,883 5/1963 Robillard 961 3,142,562 6/1964 Blake 9 61 3,152,903 10/1964 Shepard 96-64 7 NORMAN G. TORCHIN, Primary Examiner.

A. L. LIBERMAN, C. E. VAN HORN,

Assistant Examiners. 

1. AN ELECTRO-CATALYTIC PHOTOGRPAHIC RECORDING MEDIUM COMPRISING A SEMICONDUCTOR OXIDE HAVING TWO OXIDATION STATES OF DIFFERENT COLOR, SAID OXIDE BEING SUBSTANTIALLY IN THE HIGHER STATE OF OXIDATION AND DOPED WITH AN IMPURITY IN A QUANTITY TO RENDER SAID SEMICONDUCTOR OXIDE SENSITIVE TO THE INTRODUCTION OF A RELATIVELY SMALL NUMBER OF METAL IONS OF A SOLID CATALYST WITH AN ELECTRIC FIELD TO CHANGE SAID OXIDE BY CATALYTIC ACTION FROM THE HIGHER TO A LOWER STATE OF OXIDATION, AND A PHOTOCONDUCTIVE MATERIAL DISTRIBUTED OVER A SURFACE SUBSTANTIALLY COEXTENSIVELY WITH SAID OXIDE.
 8. THE METHOD OF PRODUCING A SUBSTANTIALLY PERMANENT VISIBLE PHOTOGRAPHIC RECORD CHARACTERISTIC OF A RADIATION IMAGE COMPRISING THE STEPS OF PROJECTING SAID IMAGE ON A SENSITIZED SURFACE COMPRISING A SEMICONDUCTOR MATERIAL HAVING TWO OXIDATION STATES OF DIFFERENT COLOR AND A PHOTOCONDUCTIVE MATERIAL, SAID SEMICONDUCTOR BEING SUBSTANTIALLY IN THE HIGHER STATE OF OXIDATION AND DOPED WITH AN IMPURITY IN A QUANTITY TO RENDER SAID SEMICONDUCTOR MATERIAL SENSITIVE TO THE INTRODUCTION OF A RELATIVELY SMALL NUMBER OF IONS OF A SOLID METAL CATALYST, PLACING A SOLID CATALYST SOURCE IN CONTACT WITH SAID SURFACE AND IMPRESSING AN ELECTRIC FIELD THROUGHOUT SAID SURFACE TO INJECT SAID METAL IONS OF SAID CATALYST INTO SAID SURFACE IN ACCORDANCE WITH THE CONDUCTIVITY OF SAID PHOTOCONDUCTIVE MATERIAL CAUSING SAID SEMICONDUCTOR MATERIAL TOCONVERT TO SAID LOWER OXIDATION STATE IN SELECTED AREAS THEREOF. 